Needs are developing in the control of cost and the downsizing of electronic packaging. The goals are to provide higher density, finer pitch and higher performance, all at lower cost. In the mounting of circuitry on an insulating backing, the standard in the art for interconnections or vias through the insulating backing has been the plated through hole. At this state of the art however, the plated through hole technology is encountering cost pressures and it has limits in how dense or tightly packed the circuitry can be.
The operations involved in manufacturing plated through holes or vias are expensive and are not readily adaptable to the future pad density requirements. The present plated through hole technology is approaching limits at about 20 pads or circuit node points per square centimeter. The needs of the technology indicate that a density of many times that will probably be required.
Efforts in the art to meet the density, pitch, performance and cost goals have been directed to a technique called “built up multilayer” where the circuits involve a series of superimposed laminates of foil and insulator that are interconnected using fine pitch holes that are filled with conductive paste. The technique has been extensively discussed in the literature, an example being R. Lasky, Electronic Packaging and Production, April, 1998, pages 75-78. There are many variations. In general the technique can provide a density of greater than 100 pads per square centimeter.
The built up multilayer circuitry technique requires an electrically conducting paste that will be used to fill the many small holes or vias. The paste is also referred to in the art by the term adhesive. The electrically conductive paste must be thermally and reliably compatible with the fabrication and service specifications for the places where it is to be used. The paste, in general, has metal particles suspended in a vehicle, also known in the art by the terminology resin, that can be forced into a via hole through the insulator and when subjected to a curing type cycle, during which the vehicle solidifies with the metal particles in contact with each other forming an electrical path through the via between conductors on surfaces on each side of the insulator.
One such paste is described in U.S. Pat. No. 5,652,042 where metal particles such as copper are suspended in an epoxy vehicle. The paste is squeegeed or doctor bladed into the via holes. There is a heat and pressure cycle that hardens the vehicle. The metal particles in contact with each other provide electrical interconnection.
Further work in paste technology involves introducing silver flakes into the vehicle, coating the metal particles with a low melting metal to produce metallurgical bonding of the metal particles, and, the use of a screen printable paste of silver, copper and epoxy. Problems remain with such process properties as vehicle shrinkage in connection with change of state, and the corrosion and electromigration properties of the metals, especially pure silver and copper.
In patent applications, Ser. No. 09/111,155 filed Jul. 7, 1998 and Ser. No. 09/078,043 filed May 13, 1998 a technology is described of providing and using an alloy coated metal particle which broadens the metallurgical bond forming range thereby providing metallurgical bonds not only between the particles but also to the conducting pads.
As the art progresses, the specifications will become tighter and the process windows will become narrower, so that there will be a growing need for an ever broader range of properties and selection of metal particles and vehicle formulations.